Adaptive demagnetization compensation for a motor in an electric or partially electric motor vehicle

ABSTRACT

In an electric or hybrid electric vehicle, a voltage monitor ( 102 ) is directly coupled to a traction motor ( 38 ) and/or generator motor ( 30 ) to detect a permanent magnet induced voltage within the motor at a predetermined speed and no load condition ( 300 ). A controller ( 100 ) compares the detected permanent magnet induced voltage with an expected reference voltage that represents an expected permanent magnet induced voltage at full magnetization and the predetermined speed ( 302 ). The controller produces an indication of magnetization based on the reference voltage, the detected permanent magnet induced voltage, and the predetermined speed. If the indication of magnetism reaches a predetermined threshold, the motor is made inoperable and/or a current to the motor is limited to prevent damage to components ( 306, 308, 310, 312, 314 ). Preferably, a user of the vehicle is made aware of these actions by an audible and/or visual indicator ( 308, 314 ). If available, another source of motive power is substituted for the motor that is made inoperable ( 316 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and disclosure with commonly assignedprior U.S. patent application Ser. No. 09/849,576, filed May 4, 2001 byVijay K. Garg et al., entitled “Permanent Magnet Degradation Monitoringfor Hybrid and Electric Vehicles,” the disclosure of which priorapplication is hereby incorporated by reference, verbatim and with thesame effect as though it were fully and completely set forth herein.Also, this application is related to and shares disclosure with commonlyassigned U.S. patent application Ser. No. 09/682,534, filed Sep. 7, 2001by Abbas Rafteri et al., entitled “Adaptive Demagnetization Compensationfor a Motor in an Electric or Partially Electric Motor Vehicle,” thedisclosure of which application is hereby incorporated by reference,verbatim and with the same effect as though it were fully and completelyset forth herein. Also, this application is related to and sharesdisclosure with commonly assigned U.S. patent application Ser. No.09/682,529, filed Sep. 14, 2001 by Abbas Rafteri et al., entitled“Adaptive Demagnetization Compensation for a Motor in an Electric orPartially Electric Motor Vehicle,” the disclosure of which applicationis hereby incorporated by reference, verbatim and with the same effectas though it were fully and completely set forth herein. Also, thisapplication is related to and shares disclosure with commonly assignedU.S. patent application Ser. No. 09/682,531, filed Sep. 17, 2001 byAbbas Rafteri et al., entitled “Fault Identification Due toDemagnetization for a Motor in an Electric or Partially Electric MotorVehicle,” the disclosure of which application is hereby incorporated byreference, verbatim and with the same effect as though it were fully andcompletely set forth herein.

BACKGROUNDS OF THE INVENTION

1. Field of the Invention

The present invention relates generally to hybrid electric vehicles(HEV) and electric vehicles, and specifically to compensation due topermanent magnet degradation in motors/generators in hybrid electric andelectric vehicles.

2. Discussion of the Prior Art

The need to reduce fossil fuel consumption and emissions in automobilesand other vehicles predominately powered by internal combustion engines(ICEs) is well known. Vehicles powered by electric motors attempt toaddress these needs. Another alternative known solution is to combine asmaller ICE with electric motors into one vehicle. Such vehicles combinethe advantages of an ICE vehicle and an electric vehicle and aretypically called hybrid electric vehicles (HEVs). See generally, U.S.Pat. No. 5,343,970 to Severinsky.

The HEV is described in a variety of configurations. In oneconfiguration, the electric motor drives one set of wheels and the ICEdrives a different set. Other, more useful, configurations exist. Forexample, a series hybrid electric vehicle (SHEV) configuration is avehicle with an engine (most typically an ICE) connected to an electricmotor called a generator. The generator, in turn, provides electricityto a battery and another motor, called a traction motor. In the SHEV,the traction motor is the sole source of wheel torque. There is nomechanical connection between the engine and the drive wheels. Aparallel hybrid electrical vehicle (PHEV) configuration has an engine(most typically an ICE) and an electric motor that work together invarying degrees to provide the necessary wheel torque to drive thevehicle. Additionally, in the PHEV configuration, the motor can be usedas a generator to charge the battery from the power produced by the ICE.

A parallel/series hybrid electric vehicle (PSHEV) has characteristics ofboth PHEV and SHEV configurations and is sometimes referred to as a“powersplit” configuration. In one of several types of PSHEVconfigurations, the ICE is mechanically coupled to two electric motorsin a planetary gear-set transaxle. A first electric motor, thegenerator, is connected to a sun gear. The ICE is connected to acarrier. A second electric motor, a traction motor, is connected to aring (output) gear via additional gearing in a transaxle. Engine torquecan power the generator to charge the battery. The generator can alsocontribute to the necessary wheel (output shaft) torque if the systemhas a one-way clutch. The traction motor is used to contribute wheeltorque and to recover braking energy to charge the battery. In thisconfiguration, the generator can selectively provide a reaction torquethat may be used to control engine speed. In fact, the engine, generatormotor and traction motor can provide a continuous variable transmission(CVT) effect. Further, the HEV presents an opportunity to better controlengine idle speed over conventional vehicles by using the generator tocontrol engine speed.

The generator motor and the traction motor include permanent magnets.These permanent magnets may demagnetize by accident and may degrade ordemagnetize over time due to temperature, high current ripples, powerripples, vibration and aging. The demagnetization may degrade vehicleperformance such as output power/torque and efficiency. Indeed, thedemagnetization may reach a point where safety becomes an issue. Morespecifically, demagnetization may result in less torque being availableto drive the wheels at a critical point, for example, to pass a vehicle.And, demagnetization may result in less energy being available forregenerative braking, which may adversely affect stopping distance/time.

U.S. Pat. No. 5,650,706 issued to Yamada et al. (“Yamada”) is directedto a control device for a salient pole type permanent magnet motor. Theobject of that device is to prevent torque from lowering due todemagnetization of the magnet. A magnetic flux of the permanent magnetis calculated or inferred by determining an electromotive force of thepermanent magnet in accordance with a voltage and current supplied tothe permanent magnet motor, a rotational speed of the motor, and aninductance of the permanent magnet motor. This electromotive force iscompared to a reference electromotive force representative of a fullymagnetized permanent magnet. This process is complex and cumbersome.Direct detection of demagnetization is suggested in Yamada by usingcertain sensors, such as a Hall device or a magnetoresistance element.These direct detection methods suggested in Yamada are relativelyexpensive and impact serviceability due to location of a complex sensorin the motor housing. Also, demagnetization beyond a safety limit is notmonitored and reported for safety-related actions.

Therefore, a need exists for an improved method and apparatus formonitoring and compensating for permanent magnet degradation.

SUMMARY OF INVENTION

Accordingly, an object of the present invention is to provide a monitorfor permanent magnet degradation for an electric or a hybrid electricvehicle (HEV).

Another object of the present invention is to provide a safe and directmethod for determining the magnetic flux of a permanent magnet in amotor.

Yet another object of the present invention is to determine a state ofmagnetism of a permanent magnet to adjust a torque of a motor in avehicle.

Yet another object of the present invention is to provide adaptivestrategies to compensate for permanent magnet degradation, includingprotection of components, limited operation, and notification ofpermanent magnet degradation to a user of the vehicle.

Other objects of the present invention will become more apparent topersons having ordinary skill in the art to which the present inventionpertains from the following description taken in conjunction with theaccompanying figures.

In accordance with one aspect of the present invention, a device isprovided for compensating for permanent magnet degradation in a motor.The device includes a voltage monitor that detects a permanent magnetinduced voltage within the motor at a predetermined speed and no loadcondition. The voltage monitor is coupled to a processor that receivesthe permanent magnet induced voltage and compares the permanent magnetinduced voltage to a reference voltage that reflects the permanentmagnet induced voltage for the motor with a fully magnetized permanentmagnet. The processor determines an indication of magnetism of thepermanent magnet as a function of the detected permanent magnet inducedvoltage, the reference voltage, and the predetermined speed. If theindication of magnetism reaches a predetermined threshold, the motor ismade inoperable and/or a current to the motor is limited to preventdamage to components. Preferably, an audible and/or visual indicatornotifies a user of the vehicle that the motor has been renderedinoperable or is operating in a limited mode. Preferably, another sourceof motive power, for example, another motor or an internal combustionengine or a combination of these, is substituted for the motor that ismade inoperable.

In accordance with another aspect of the present invention, a method isprovided for adapting to permanent magnet degradation in a motor of avehicle. First a permanent magnet (PM) induced voltage of a motor isdetected. Preferably, the permanent magnet induced voltage is detectedby inducing a voltage in coils wrapped around the stator teeth of amotor. The voltage is induced at a predetermined speed by the rotationof a rotor that includes the permanent magnets. The detected permanentmagnet induced voltage is compared to a reference voltage that reflectsfull magnetism of the permanent magnets at the predetermined speed. Anindication of magnetism of the permanent magnets is produced as afunction of the detected permanent magnet induced voltage, the referencevoltage and the predetermined speed. If the indication of magnetismreaches a predetermined threshold, the motor is made inoperable and/or acurrent to the motor is limited to prevent damage to components.Preferably, an audible and/or visual indicator notifies a user of thevehicle that the motor has been rendered inoperable or is operating in alimited mode. Preferably, another source of motive power, for example,another motor or an internal combustion engine or a combination ofthese, is substituted for the motor that is made inoperable.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing objects, advantages, and features, as well as otherobjects and advantages, will become apparent with reference to thedescription and figures below, in which like numerals represent likeelements and in which:

FIG. 1 is a block diagram illustrating a hybrid electric vehicle (HEV)configuration in accordance with a preferred embodiment of the presentinvention.

FIG. 2 is a block diagram of a transaxle management unit in accordancewith a preferred embodiment of the present invention.

FIG. 3 is a cross sectional view of a motor in accordance with apreferred embodiment of the present invention.

FIG. 4 is a flow diagram illustrating a method of detecting and adaptingto permanent magnetism degradation in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to electric vehicles and, moreparticularly, hybrid electric vehicles (HEVs). FIG. 1 illustrates aparallel/series hybrid electric vehicle (powersplit) configuration inaccordance with the present invention.

In the HEV of FIG. 1, a planetary gear set 20 mechanically couples acarrier gear 22 to an engine 24 via a one way clutch 26. The planetarygear set 20 also mechanically couples a sun gear 28 to a generator motor30 and a ring (output) gear 32. The generator motor 30 also mechanicallylinks to a generator brake 34 and is electrically linked to a battery36. A traction motor 38 is mechanically coupled to the ring gear 32 ofthe planetary gear set 20 via a second gear set 40 and is electricallylinked to the battery 36. The ring gear 32 of the planetary gear set 20and the traction motor 38 are mechanically coupled to drive wheels 42via an output shaft 44.

The planetary gear set 20 splits the engine output energy into a seriespath from the engine 24 to the generator motor 30 and a parallel pathfrom the engine 24 to the drive wheels 42. Engine speed can becontrolled by varying the split to the series path while maintaining themechanical connection through the parallel path. The traction motor 38augments the engine power to the drive wheels 42 on the parallel paththrough the second gear set 40. The traction motor 38 also provides theopportunity to use energy directly from the series path, essentiallyrunning off power created by the generator motor 30. This reduces lossesassociated with converting energy into and out of chemical energy in thebattery 36 and allows all engine energy, minus conversion losses, toreach the drive wheels 42.

A vehicle system controller (VSC) 46 controls many components in thisHEV configuration by connecting to each component's controller. Anengine control unit (ECU) 48 connects to the engine 24 via a hardwireinterface. The ECU 48 and VSC 46 can be housed in the same unit, but arepreferably separate controllers. The VSC 46 communicates with the ECU48, as well as a battery control unit (BCU) 50 and a transaxlemanagement unit (TMU) 52 through a communication network, such as acontroller area network (CAN) 54. The BCU 50 connects to the battery 36via a hardwire interface. The TMU 52 controls the generator motor 30 andtraction motor 38 via a hardwire interface. More specifically, TMU 52includes a controller that executes a stored program to determine thetorque of generator motor 30 and traction motor 38. Also, in accordancewith the present invention, TMU 52 detects and stores an indication ofthe magnetization of permanent magnets in generator motor 30 andtraction motor 38. In particular, a voltage sensor incorporated ingenerator motor 30 and a voltage sensor in traction motor 38 determine avoltage that is proportional to the magnetization of permanent magnetsin generator motor 30 and traction motor 38, as described below. Also,in accordance with the present invention, TMU 52 controls motor torqueand current, motor operation, and initiates warnings to a user of thevehicle.

FIG. 2 is a block diagram of a portion of transaxle management unit 52shown interfaced to generator motor 30 and traction motor 38 inaccordance with a preferred embodiment of the present invention. TMU 52preferably includes a controller 100, a voltage monitor 102, a voltagemonitor 104, an inverter 106 and an inverter 108. Inverter 106 iscoupled to traction motor 38 to provide a three-phase AC current totraction motor 38. The three-phase AC current is derived from a DCcurrent from battery 36. Similarly, inverter 108 is coupled to generatormotor 30 to provide a three-phase AC current to generator motor 30. Thethree-phase AC current is also derived from a DC current from battery36. Inverter 106 an inverter 108 are coupled to controller 100 such thatcontroller 100 provides input signals to inverters 106,108 to determinea current provided to generator motor 30 and traction motor 38,respectively. In accordance with the present invention, a voltagemonitor 102 is coupled to traction motor 38 to determine a permanentmagnet induced voltage of traction motor 38. Similarly, a voltagemonitor 104 is coupled to generator motor 30 to determine permanentmagnet induced voltage in generator motor 30. The permanent magnetinduced voltages from traction motor 38 and generator motor 30 are usedby controller 100 to determine a state of the permanent magnetscontained within traction motor 38 and generator motor 30. Based on thestate of the permanent magnets in traction motor 38 and generator motor30, controller 100 determines a current provided by inverters 106, 108determines whether the traction motor 38 or generator motor 30 areoperable; and initiates warnings to a user.

Controller 100 preferably includes a processor 110 and a memory 112.Processor 110 comprises one or more microprocessors, micro-controllers,or the like. Controller 100 preferably executes a stored program todetermine, store and transmit an indication of the state of magnetism ofthe permanent magnets contained within generator motor 30 and tractionmotor 38. Also, controller 100 preferably executes a stored program todetermine actions to be taken based on the state of magnetism of thepermanent magnets contained within generator motor 30 and traction motor38. Most preferably, memory 112 includes a non-volatile memory componentthat stores an indication of the state of magnetism of the permanentmagnets in generator motor 30 and traction motor 30.

Voltage monitors 102,104 preferably include a voltage sensor 114 and avoltmeter 116. Voltage sensor 114 is directly coupled to its respectivemotor to determine a permanent magnet induced voltage at a predeterminedspeed of the motor 38. The voltmeter 116 provides the voltage fromvoltage sensor 114 to controller 100 for use in determining the state ofmagnetism of the permanent magnets in generator motor 30 and tractionmotor 38. Preferably, the voltmeter is housed external to the motor.Most preferably, the voltmeter is hardware available on the vehicle thatis reused for the magnetization monitoring function, which function isonly required periodically.

FIG. 3 is a sectional view of generator motor 30 including a preferredvoltage sensor in accordance with the present invention. A similararrangement is preferred for traction motor 38. Generator motor 30includes a rotor 200 and a stator 202. Permanent magnets 208 are mountedwithin rotor 200. The motor windings 204 (as exemplary shown between twostator teeth) are wrapped around the teeth 205 in slots 206 in stator202 in the traditional manner. In accordance with the invention, asensor coil 210 is wrapped around the teeth 205 in slots 206 in stator202. As shown in FIG. 3, sensor coil 210 is preferably located adjacentrotor 200 at an edge of the teeth 205 closest to a gap between stator202 and rotor 200. Preferably, sensor coil 210 comprises a wire having avery high gauge and a few turns. The sensor coil 210 is coupled tovoltmeter 116 and serves as a voltage sensor 114. Sensor coil 210 isused to determine a permanent magnet induced voltage in generator 30.More specifically, when no current is running through motor windings204, a voltage is induced in sensor coil 210 due to the rotation ofrotor 200 and a magnetic field generated by permanent magnets 208. Thisvoltage is sensed by voltmeter 116 and is transmitted to controller 100.

FIG. 4 is a flow diagram illustrating a method for determining andcompensating for permanent magnet degradation in a motor in accordancewith the present invention. The method is described below with referenceto the preferred embodiments described above in FIGS. 1-3.

First, the permanent magnet induced voltage of the motor is determined(300). In the preferred embodiment, this is accomplished by inducing avoltage in sensor coil 210 during a period of time when no current isflowing in the motor windings; i.e., there is no load. Preferably,voltmeter 116 quantifies the voltage induced in the sensor coil 210. Theno load condition occurs when there is zero current in the statorwindings of the motor. For example, the no load condition occurs whenthe vehicle is at idle, for example, stopped at a stop light, and also,when the vehicle is at cruising speed and there is no current in themotor windings. Another exemplary no load condition occurs when thegenerator motor is not supplying any torque to the wheels or receivingtorque from the engine to charge the batteries. The PM induced voltageis preferably induced by the rotation of rotor 200, including permanentmagnets 208. This causes a magnetic field that induces the voltage inthe sensor coil. Most preferably, rotor 200 is rotated at apredetermined speed and the inverter contacts that supply current to themotor are opened during permanent magnet induced voltage measurement.The TMU 52, and more specifically, controller 100 determines when tomeasure the permanent magnet induced voltage in light of the state ofthe vehicle, which state is preferably obtained via controller areanetwork 54 or any other suitable means.

The permanent magnet induced voltage is proportional to the magneticfield (flux) and the speed of rotation of the rotor. Hence, the strengthof the permanent magnet is readily obtained where the speed andpermanent magnet induced voltage are known.

After the permanent magnet induced voltage is detected, the permanentmagnet induced voltage is compared to a reference voltage that reflectsa permanent magnet induced voltage at no demagnetization and the samepredetermined speed at which the permanent magnet induced voltage isdetected (302). That is, the reference voltage is the value expected forthe permanent magnet induced voltage if the permanent magnet is fullymagnetized. Preferably, the reference voltage is stored in TMU 52. Anydifference between the reference voltage and the detected permanentmagnet induced voltage is used to determine an indication of the amountof degradation of the permanent magnet. This indication is preferablystored in a nonvolatile memory for further reference (304). Also, theindication of magnetic strength is compared to a first threshold todetermine if the permanent magnet has reached a point of degradationwhere additional precautions should be taken (306). Most preferably, ifthe magnetic strength is below a predetermined first threshold, anindication is made to a user of the vehicle, for example, through anaudible or visual indication that is transmitted via controller areanetwork 54 (308). Also, the current to the motor is limited to an amountthat prevents damage to components of the vehicle (308) and/or the TMU52 is calibrated to more accurately drive the inverter to force themotor to provide the torque required. Most preferably, the firstthreshold is chosen such that at least limited operation of the vehicleis possible. During a period of continued limited operation, permanentmagnet degradation along with other motor parameters, such astemperature, are monitored (310). The results from further monitoring(310), are compared to a second threshold (312). This threshold isalternatively a level of magnetism, a certain temperature, or anothermonitored parameter. If the second threshold is not met (312), thenmonitoring continues (310). If the second threshold is met (312), thensubsequent motor operation is suspended and a user of the vehicle iswarned with an audible or visual indicator (314). Where another sourceof motive power is available, operation of the wheels of the vehicle isswitched to that motive source (316). For example, in the preferredembodiment of FIG. 1, if the generator motor 30 is made inoperable dueto permanent magnet degradation, then wheels 42 are operated undercontrol of traction motor 38. Alternatively, if traction motor 38 ismade inoperable due to permanent magnet degradation, then wheels 42 areoperated under control of generator motor 30 and engine 24. Mostpreferably, if the traction motor 38 is inoperable, generator motor 30is first used to bring wheels 42 to a certain speed, and then engine 24is activated to provide additional motive force via a smooth transition.

In preferred alternatives to the method described above with respect toFIG. 3, the first and second thresholds are varied to be the same or oneor the other is ignored. For example, steps 306, and 308 may beeliminated such that a threshold measure of magnetism (312) immediatelycauses the motor to be made inoperable (314). Alternatively, steps 310,312, 314 and 316 may be eliminated if magnet degradation does not resultin inoperability or unsafe conditions.

As discussed above, the present invention provides a simple andeffective method of determining the state of magnetism of a permanentmagnet in a motor of a vehicle. Advantageously, the state of magnetismis compared with a safety threshold and an indication of safety problemsis made available to a user of the vehicle. Also, the state of magnetismis used to calibrate a torque from the motor, limit a current to themotor, suspend motor operation, or switch to an alternative motiveforce.

The above-described embodiments of the invention are provided purely forpurposes of example. Many other variations, modifications, andapplications of the invention may be made.

What is claimed is:
 1. A hybrid electric vehicle comprising: a tractionmotor; a generator motor; a first voltage monitor coupled to thetraction motor to determine a first permanent magnet induced voltage ofthe traction motor; a second voltage monitor coupled to the generatormotor to determine a second permanent magnet induced voltage of thegenerator motor; a controller that: compares the first permanent magnetinduced voltage with a first reference voltage that reflects an expectedpermanent magnet induced voltage for the traction motor when a permanentmagnet of the traction motor is fully magnetized; compares the secondpermanent magnet induced voltage with a second reference voltage thatreflects an expected permanent magnet induced voltage for the generatormotor when a permanent magnet of the generator motor is fullymagnetized; determines a state of magnetism of the permanent magnet ofthe traction motor based on the first permanent magnet induced voltage,the first reference voltage and a predetermined speed at which the firstpermanent magnet induced voltage is determined; determines a state ofmagnetism of the permanent magnet of the generator motor based on thesecond permanent magnet induced voltage, the second reference voltage,and a predetermined speed at which the second permanent magnet inducedvoltage is determined; and adjusts a current to at least one of thegenerator motor and the traction motor based on at least one of thestate of magnetism of the generator motor and the state of magnetism ofthe traction motor.
 2. The vehicle of claim 1 wherein the first voltagemonitor comprises a sensor coil that detects the first permanent magnetinduced voltage that is induced by rotation of a rotor that includes thepermanent magnet of the traction motor and wherein the second voltagemonitor comprises a sensor coil that detects the second permanent magnetinduced voltage that is induced by rotation of a rotor that includes thepermanent magnet of the generator motor.